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ANATOMIC CONSIDERATIONS

In neonates and infants, the conus medullaris is located at L3, which is more caudal than in adults (L1). Because of the difference in the rates of growth between the spinal cord and the bony vertebral column, the conus medullaris reaches L1 at approximately 1 year of age. Thus, lumbar puncture for subarachnoid block in neonates and infants should be performed at L4-L5 or L5-S1 so as not to injure the spinal cord. The midline approach is preferred over paramedian because the vertebral laminae are poorly calcified in neonates and infants.

The sacrum is narrower and flatter in neonates. This difference affects the approach to the subarachnoid space from the caudal canal. It is much more direct in neonates than in adults. The needle must not be advanced deeply in neonates because dural puncture is much more likely.

The distance from the skin to the subarachnoid space in neonates is approximately 1.4 cm, progressively increasing with age. The ligamentum flavum is much thinner and less dense in children than adults, which makes it more difficult to detect engagement of the epidural needle and results in unintended dural puncture.

Cerebrospinal fluid (CSF) volume per percentage of body weight is greater in infants than in adults. This may account for the comparatively larger doses of local anesthetics required for surgical anesthesia with subarachnoid block.

A caudal block may be contraindicated in the presence of a deep sacral dimple because this may indicate the presence of spina bifida occulta, thus greatly increasing the probability of dural puncture.

PHYSIOLOGIC CONSIDERATIONS

Cardiovascular system—Subarachnoid and epidural blockade in children is characterized by hemodynamic stability even if the block reaches the level of the upper thoracic dermatomes. The heart rate is preserved because of parasympathetic activity and modulating the heart rate appears to be attenuated in infants. The attenuated vagal tone allows the heart rate to compensate for alterations in peripheral vascular tone.

Respiratory system—Central neuraxial blockade can affect the respiratory mechanics of the chest wall and diaphragm by diminished activity of the intercostal muscles. In infants and young children, the chest walls are very compliant due to limited ossification of the ribs. They rely on the diaphragm for the maintenance of tidal volume more than adults. Studies of infants have demonstrated that during rapid eye movement and deep sleep, paradoxical inward chest wall motion occurs commonly and increases as the force of diaphragmatic excursion increases. When high thoracic levels of motor blockade is achieved during spinal anesthesia in infants, outward motion of the lower rib cage decreases and paradoxical motion of the lower rib cage occurs. The diaphragmatic contribution to respiration is increased. This suggests a shift in respiratory workload from the rib cage to the diaphragm in compensation for the loss of the intercostal muscle contribution to breathing. The ability of the diaphragm to compensate for the loss of contribution of the rib cage to breathing is ...